You can deduct any cost you incur that's an ordinary and necessary expense of farming on Schedule F to reduce the profit—or increase the loss—on which you'll owe taxes.
Although I don't see a specific expense line for 'corn check off fees' on Schedule F, you can include it as a Miscellaneous Expense at the end of the expense section in TurboTax.
Click this link for detailed instructions from the IRS on Schedule F.
This article on Schedule F may also be helpful.
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